Why does the metal connector between our gas furnace and the chimney keep rotting?

I’m going to guess that you have a newer furnace. If that’s true, you’re probably getting condensation in the flue. Older furnaces didn’t have that issue. The hot flue gas they produced literally shot out of the chimney – too hot and moving too quickly to condense. Although that method kept the flue clean, there was a glaring problem: all of that hot gas was wasted energy. So furnace manufacturers, obeying mandated fuel-efficiency standards, now build appliances that use far less fuel. This results in a smaller volume of exhaust at a cooler temperature, giving the water vapour formed in the combustion process a greater chance of condensing in the flue. If a chimney is on the end of the house, with exposure to cold air on three sides, the problem is even worse. The resulting condensate can erode the chimney flue, rot the metal connector, and even damage the furnace.

“Diagnosis requires a competent contractor to check for condensation, combustion efficiency and adequate combustion air,” says HVAC expert Pat Porzio. If the furnace is designed for direct venting, the solution may be to bypass the chimney, either by venting out the sidewall or through another roof vent. In some cases, the contractor can install a power vent – a blower to move the exhaust gas out before condensation can form. An insulated chimney-flue liner may solve the problem, too. Once a part works loose, creating an airspace between it and its mounting surface, the looseness acts as a force multiplier. You tighten it and it loosens even more. The cycle repeats until the part comes completely undone or breaks. The moral? Tighten something as soon as it comes loose, otherwise it will only get worse.

When tightening, check if the threads are stripped, if anything was over- or under-tightened, or if the wrong size or type of fastener was used. There’s almost always a good reason that tight things become loose.

The first thing to do in your repair is ensure that the mounting screws for the brackets are securely in place. You may be tempted to try to fix a stripped-out hole by inserting a piece of copper wire or a glue-covered golf tee, but this is not the place to do it. You need to reach a solid fastening point. If that doesn’t work, you’ll need a thicker and larger mounting bracket. You might even sneak in one structural screw (a hybrid fastener, somewhere between a lag screw and a wood screw) at each bracket location. Even if you have to slightly drill out the screwmounting hole to accept the structural screw, it’d be worth it to fix this problem for good. The problem is saponification. The primer and paint you applied 30 years ago weathered away and let in moisture, which formed zinc oxide on the door’s surface. That oxide combines with oils that leach out of any new layers of paint you apply to create a crude form of soap. When you add more paint to it, the soap attacks the paint film and loosens it. Plus, when the Sun beats down on the damp door, any moisture under the paint turns into vapour and forces its way through the topcoat.

The solution is to strip as much of the paint as possible. Sand smooth any rough edges where the paint blistered and was scraped or brushed off. Next, apply a “prep and etch” product, which is a diluted form of phosphoric acid. You brush it on, let it set for 15 to 30 minutes, rinse thoroughly, and then let the door dry. As soon as possible after that, prime the door with a high-performance 100 per cent acrylic primer formulated for surfaces that might have an adhesion problem, such as vinyls, plastics and galvanised metals. Follow that with two coats of exterior acrylic topcoat. PM